The invention relates to a computed tomography method which includes the steps of:
generating, using a radiation source, a conical radiation beam that traverses an examination zone or an object present therein,
generating relative motions between the radiation source on the one side and the examination zone or the object on the other side, which relative motions include a rotation about an axis of rotation along a first, closed trajectory and along at least a second trajectory which is identical to the first trajectory but offset in the direction of the axis of rotation,
acquiring, while using a detector unit, measuring data that is dependent on the intensity in the radiation beam to the other side of the examination zone during the relative motions,
reconstructing the absorption distribution in the examination zone.
The invention also relates to a computed tomography apparatus as well as to a computer program for controlling such a computed tomography apparatus.
The use of a conical radiation beam instead of a fan-shaped beam offers the advantage that a larger part of the examination zone (measured in the direction of the axis of rotation), or of the object present therein, can be covered and reproduced by means of a single rotation along the closed trajectory. Should an even larger part of the examination zone be imaged, such a rotation should be succeeded at least once by a relative motion in the direction of the axis of rotation and the examination zone should be irradiated and imaged from a second trajectory. The shift must be selected to be so large that imaging can take place without gaps.
According to a method that is known from German patent application 19 843 812.5 (PHD 98-111), the shift may be so large that the voxels in an intermediate region are not continuously exposed to X-rays neither from the one trajectory nor from the other trajectory. The reconstruction of the absorption distribution in this intermediate region is then realized in that only measuring data which is associated with a radiation angle range of only 180xc2x0 (relative to the voxel) is taken into account for each voxel. It is a drawback of this method that the absorption distribution in the intermediate region can be reconstructed only with a less favorable signal-to-noise ratio and with additional artefacts, that is, in comparison with the absorption distribution in the regions that are continuously exposed to radiation during the rotation of the radiation source along the trajectories.
Granted, this drawback could be avoided by choosing the shift between the trajectories to be so small that the regions of the object that are continuously exposed to radiation from one of the trajectories directly adjoin one another. This leads to an increased radiation load when the method is used for a medical examination, but at least to increased thermal loading of the radiation source. Moreover, the acquisition time required for scanning a part of the examination zone that is selected in the direction of the axis of rotation is thus prolonged.
It is an object of the present invention to provide a method of the kind set forth such that on the one hand a suitable image quality is obtained and on the other hand a low dose load or short acquisition time for covering a given part of the examination zone.
This object is achieved in accordance with the invention by selecting the distance between the trajectories to be such that:
on the one hand voxels that are temporarily not exposed to any radiation during the two relative motions are present in an intermediate region between the trajectories, and
on the other hand the radiation source projects both trajectories continuously on the detector unit in such a manner that the projections intersect the lateral edge of the area of the detector unit that is used for the acquisition of measuring data, and
the absorption distribution in the intermediate region being reconstructed while taking into account measuring data acquired during the two relative motions.
Thus, in conformity with the invention there are not only the regions whose absorption can be completely measured from one of the trajectories but also an intermediate region in which such measurement is not possible, because the voxels in this region are not irradiated by the radiation source during the entire rotation along the trajectory. The invention is based on the recognition of the fact that the angular ranges wherefrom a voxel in the intermediate region is irradiated from both trajectories together amount to at least 360xc2x0 in the case of the stated selection of the distance of the trajectories, so that the absorption distribution in the intermediate region can also be reconstructed with an image quality which is comparable to that attained for the regions whose absorption distribution is reconstructed from the measuring data acquired along a single trajectory, provided that measuring data from both trajectories are used for the reconstruction.
Preferably, the distance between the neighboring trajectories is chosen in conformity with claim 2. This choice of the distance is optimum. When a smaller distance is chosen, the radiation load is increased or the measuring time is prolonged whereas in the case of a larger distance (the projection of one trajectory on the detector unit then no longer intersects the lateral edge of the area of the detector unit that is used for the acquisition of measuring data) it is no longer possible to reconstruct the absorption distribution in the intermediate region without loss of image quality.
Claim 3 discloses a preferred reconstruction method that involves an amount of calculation work that is small in comparison with other methods and yields a very high image quality. The reconstruction of the absorption in the intermediate region then utilizes filtered data of groups acquired from different trajectories. This reconstruction method is known per se from German patent application . . . (PHD 98-123) or from the article by Graxcex2, Kohler, Proksa xe2x80x9c3D cone-beam CT reconstruction for circular trajectoriesxe2x80x9d in Phys. Med. Biol 45 (2000) 329-347. The version of the method that is disclosed in claim 4 enables a particularly simple reconstruction.
Claim 5 discloses a computed tomography apparatus for carrying out the method in accordance with the invention. The preferred embodiment disclosed in claim 6 ensures, in conjunction with the reconstruction method claimed in claim 3 and using the distance defined in claim 2, that the edge of the radiation beam and the projection of the one trajectory on the detector unit coincide. In that case each voxel between the two trajectories receives exactly as much radiation as required for the desired image quality.
Claim 7 defines a computer program for controlling a computed tomography apparatus as claimed in claim 5.